very easily jealous.
but the period of how long she's known the guy matters. Meaning like you described, if she just met him and they talked and stuff for a brief moment, and he gave her attention first and then turned to another chick, shed be slightly jealous but would easily move on and may be won't even look back at the guy. However, if youve been friends with a guy for a while now, and flirting is daily, and you grow on that person, and then one day out of nowhere he hits on some other chick in front of you, the degree of jealousy is more intense followed by anger, pms, bitching ignoring and cold-heartedness towards the guy.
NEVER EVER flirt with another grl when uve already given signs to someone else initially. especially in front of her.
women want security from men. They want something meaningful and worthy even if it doesn't become into a long relationship, most women want a guy theyve dated who was good to her and liked her and they had good times. no one wants to be treated like shit or used and be played with, and to be honest, would you as a guy, or any guy, wanna be remembered as a douchebag or jerk or asshole in a girls memory?